U.S. Pat. No. 5,437,811 issued Aug. 1, 1995 to Doane et al. discloses a light-modulating cell having a chiral nematic liquid crystal (cholesteric liquid crystal) in polymeric domains contained by conventional patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Chiral nematic material has two stable states and can maintain one of the stable states in the absence of an electric field.
U.S. Pat. No. 5,251,048 issued Oct. 5, 1993 to Doane et al., and U.S. Pat. No. 5,644,330 issued Jul. 1, 1997 to Catchpole et al. disclose various driving methods to switch chiral nematic materials between its stable states. However, the update rate of these displays is far too slow for most practical applications. Typically, the update rate was about 10-40 milliseconds per line. It would take a 10-40 seconds to update a 1000 line display.
U.S. Pat. No. 5,748,277 issued May 5, 1998 to Huang et al., and U.S. Pat. No. 6,154,190 issued Nov. 28, 2000 to Yang et al. disclose fast driving schemes for chiral nematic displays, which are called dynamic drive schemes. The dynamic drive schemes generally comprise a preparation step, a pre-holding step, a selection step, a post-holding step, and an evolution step. These fast driving schemes require very complicated electronic driving circuitry. For example, all column and row drivers must output bi-polar and multiple level voltages. During the image writing, due to a pipeline algorithm used with the drive schemes, there is an undesirable black bar shifting over the frame.
U.S. Pat. No. 6,268,840 B1 issued Jul. 31, 2001 to Huang, discloses a unipolar waveform drive method to implement the above-mentioned dynamic driving schemes. However, because the amplitude of voltages required in the preparation step, the selection step, and the evolution step are distinct, both column and row drivers are required to generate multilevel unipolar voltages, which is still undesirable.
Kozachenko et al. (Hysteresis as a Key Factor for the Fast Control of Reflectivity in Cholesteric LCDs, Conference Record of the IDRC 1997, pp. 148-151), Sorokin (Simple Driving Methods for Cholesteric Reflective LCDs, Asia Displays 1998, pp. 749-752), and Rybalochka et al. (Dynamic Drive Scheme for Fast Addressing of Cholesteric Displays, SID 2000, pp. 818-821; Simple Drive scheme for Bistable Cholesteric LCDs, SID 2001, pp. 882-885) proposed so called U/√{square root over (2)} and U/√{square root over (3/2)} dynamic drive schemes requiring only 2-level column and row drivers, which output either U or 0 voltage. These drive schemes do not produce undesirable black shifting bars, instead, they cause the entire frame to go black during the writing. However, as their names suggest, they can be applied only to those cholesteric liquid crystal displays with very specific electro-optical properties, such as Uholding=Uevolution=U /√{square root over (2)} for the U/√{square root over (2)} dynamic drive scheme, or Uholding=Uevolution=U/√{square root over (3/2)} for the U/√{square root over (3/2)} dynamic drive scheme, where Uholding and Uevolution are effective voltages (root mean square voltages) of their holding step and evolution step, respectively. Because of this limit, many cholesteric liquid crystal displays either cannot be driven by these schemes, or can be driven only by compromising contrast and brightness.
Another problem with these drive schemes is data pattern dependent defects. Namely, the effective selection time varies depending on the nonselected pixel voltages preceding and following a selected row, thus the reflective state of a pixel changes in an undesired way. There is a need therefore for an improved dynamic drive scheme that eliminates data pattern dependent defects in a displayed image.